Just define the image seperately and add it on top of the button, or make the button clear and use an image as a background. I do not think it is possible to actually define the image within the button class. I could be wrong i haven't tried it. But I have made dialogs with images as buttons by using the image as a background and placing invisible buttons on top of the image.
I will provide a quick example, not sure if it will meet your needs or not.